The proposed research will further our understanding of how a eukaryotic cell synthesizes ribosomes in an efficient and timely manner. Ribosome biogenesis is a complex process that is necessary for both cell maintenance and growth. NOP4 is an essential yeast gene that has been implicated in ribosome biogenesis. Specifically, it mediates pre-rRNA processing of the 60S ribosomal subunit, but its exact role in processing is unclear. Nop4p is a protein capable of binding RNA. Therefore, I propose to identify the specific molecules that interact with Nop4p in its role as an essential mediator of pre-rRNA processing. Methods such as the three-hybrid system and synthetic lethality will allow me to identify both RNA and protein molecules that interact with Nop4p. After identification of such molecules, I will be able to propose a model of Nop4p's function in ribosome biogenesis.